1. Field of the Invention
The present invention relates generally to devices for the display of color information and, more particularly, to multi-color segmented or matrix type displays in which a finite number of discretely addressable picture elements (pixels) are activated in appropriate combination to form a full-color image.
2. Description of the Related Art
Previous segmented or matrix display technologies used for generating full-color alphanumeric, graphic and/or television type video images have relied on additive color synthesis via high-density arrangements of small red (R), green (G), and blue (B) primary color pixels.
Color encoding has become a common feature in visual information displays. Although many types of color display systems and applications presently exist, there are many other potentially useful applications of color which have not been developed due to limitations in existing color display technology. Virtually all existing color displays are additive color systems, in that full color is produced by either the spatial integration of very small primary color points (i.e., very small R, G, and B pixels) or the temporal integration of sequentially presented image fields of alternating primary colors.
Both of these additive approaches to color synthesis have significant limitations. Spatial additive color synthesis requires high pixel density or resolution, since the projected angle substended by of small primary color elements (i.e., R, G, and B pixels) must be encompassed within the spatial integration zones of the human visual system. If primary color elements are too large, then complete color synthesis will fail to occur and color fringes or patterns will be apparent in the image. The requirement for three "populations" of spatially separated primary color elements to produce a full-color image, as in the shadow-mask cathode ray tube, results in a reduction of available image sampling resolution of the display device. For applications requiring full color and very high image resolution, such as systems for the display of sensor video information, spatial additive approaches to color synthesis are generally not feasible due to the resultant losses in image sampling resolution. In addition, many applications for color information displays require only low image resolution, such as color-coded alphanumeric or symbolic displays. For low-resolution displays, spatial additive color technology is generally not appropriate since relatively high pixel resolution or density is required for adequate color synthesis even though image resolution requirements are substantially lower. High pixel density usually incurs high cost, and many potentially useful applications of color in low resolution displays remain undeveloped due to the relatively high cost of spatial additive color display technology.
Temporal color synthesis does not rely on three "populations" of spatially separated R, G, and B pixels to produce a full-color image, but rather achieves color synthesis by rapid sequential alternation of primary color images. This approach to color synthesis does not degrade image resolution, as does spatial color synthesis. Full color control is effectively achieved at each individual image pixel. Temporal synthesis is generally implemented by a broad-band image forming source passing light sequentially in time through three color filters (typically R, G, and B). The image forming source must be synchronized with the three color filters such that appropriate parts of the image with an intended color are displayed while the respective filter or filters are in front of the image forming source. The most popular implementations of such "frame-sequential" color display systems are typified by the use of a cathode ray tube with a broad-band phosphor (i.e., emitting white light) as the image forming source and a rotating color wheel containing R, G, and B filters as the color rendering component. More recently, the color wheel has been replaced by a non-mechanical component consisting essentially of a liquid crystal (LC) switchable optical polarizer and several layers of polarized color filter films.
The disadvantages of color display systems which utilize temporal color synthesis (i.e., frame-sequential color mixing) are rooted in the fact that, in such systems, the individual primary color image fields are separated in time and are only present for one third of the total display viewing period. Since three color image fields must be presented in the same amount of time as a single field in a spatial additive color display or a monochromatic display, frame-sequential displays require an extremely high system bandwidth in order to produce a full-color image at a refresh rate high enough to minimize observable flicker. Even with high system bandwidths and full-color refresh rates equivalent to monochromatic or spatial additive color displays (i.e., three times the refresh rates of non-frame-sequential displays), frame-sequential color displays are prone to image flicker due to the luminance modulation existing between sequential color image fields. A more important limitation of the temporal synthesis approach to color mixture, however, is that mixture colors are often observed to smear or separate into their individual primary color image components during motion of either the displayed image or the observer's eyes.
A need has therefore been felt for color display apparatus that overcomes the problems created by the use of additive techniques.